Miss distance indicator



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4 Sheets-Sheet 2 Filed Dec. 14, 1961 NA-El mc l F Alxmynm July 7, 1964 P, GIRAULT 3,140,488

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MISS DISTANCE INDICATOR Filed Dec. 14, 1961 4 Sheets-Sheet 4 AV E H/Fb t F I 6.5

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12 L F LIF Amphfier Am hfier Del'ccl'ar Del'ecl'or 51 52 Fl 60 7 Clipping Clipping Amplifier Amplifier nited atet fiice 3,140,438 Patented July 7, 1964 The present invention relates to systems for instantly measuring the distance between two objects, at least one of which is moving.

The knowledge of this distance may be of great importance, for example, when one object is a target and the other a missile which carries a proximity fuze which is to be detonated or fired at a predetermined distance from the target.

In a system according to the invention for measuring the distance between two objects, at least one of which is moving, there are provided on one of these objects: two transmitters respectively transmitting ultra-high frequency signals on two frequencies close to each other; a receiver for directly receiving said two signals and for re ceiving the respective echoes of said signals transmitted by said other object; means for detecting the respective Doppler frequency signals of said echo signals; means for deriving respective pulses each time a Doppler signal is equal to zero; and AND-gate having two inputs respec tively for receiving said pulses and an output; and a counter coupled to said output and thus controlled by said AND-gate for starting upon first simultaneous reception of said two pulses, and stopping upon second simultaneous reception of said pulses.

The invention will be best understood from the following description and appended drawings, wherein:

FIG. 1 is an explanatory diagram of the invention;

FIGS. 2a, 2b and 3 are three further diagrams showing the operation of the system according to the invention;

FIG. 4 is a block diagram of a system according to the invention;

FIG. 5 shows the signals occurring at the input of an element of the arrangement shown in FIG. 4; and

FIGS. 6 and 7 are two further embodiments of the system according to the invention.

The invention makes use of the Doppler eifect.

It will be assumed in the following that the relative motion of a target C and a missile E with respect to each other may be considered as a displacement, wherein the target is stationary and the missile describes a rectilinear trajectory in the vicinity of the target.

If a transmitter carried by the target transmits a pure continuous wave with a frequency f this will provide at the target, as shown in FIG. 1, an electrical field vector A C, which is directly due to this transmission. Assuming the phase of this vector to be the origin, a further field vector E will exist at the target due to reflection of the transmitted wave on the missile, this vector being phaseshifted by an angle If a receiver aerial is carried by the target, in addition to the transmitter aerial, it will collect a field vector R, which is the geometric sum of C and E, the phase-shift o between vectors C and B being given by the formula where r is the distance between the missile and the target and c the velocity of the electromagnetic waves.

If the missile has a certain velocity relative to the .9 target, r varies. In fact, E has an amplitude which varies but slowly with respect to the variation of phase Vector R is a maximum for K K) D=2Km and 2T= and a minimum for =(2K+1)1r, and T= i Where K is an integer and A the operating wave-length.

The Doppler signal is obtained at the receiver by the detection of signal R. This low-frequency signal, after elimination of the DC. component, corresponding to vector C of a constant amplitude and phase, is a DC.

signal corresponding to vector E. The amplitude of this signal may be considered to be constant in the vicinity of the target. This Doppler signal D is shown in FIG. 2a, which illustrates the Variations hereof as a function of time. This signal is of the form cos Q(t)t It passes through a maximum at time 1 corresponding to q =2K, then through a minimum at time t corresponding to =(2K+1)1r and is cancelled for 9 being the angular speed due to the Doppler effect.

In other Words, each time the path of the missile crosses a sphere, the radius of which is given by the following relation:

correspondingto cos S2t=0, the Doppler signal is cancelled. Since the radius r of such a sphere is given by Accordingly The family of spheres having their respective radii equal to are shown FIG. 3. The knowledge of the number of spheres of radius r which have been crossed by the missile, counting from a point located at a fixed distance from the target, and until its passage at the minimum distance from the target will thus make it possible to determine, to within where n is an integer, the Doppler signals corresponding to frequencies F and F will cancel simultaneously when the missile is at a distance corresponds, for one of the vibrations, to

2n+ we;

and for the other, to

M (Z g In terms of wave lengths, this distance corresponds to a shift of a quarter-wave length, between the echo signals.

There are other spheres corresponding to a simultaneous cancellation of the Doppler signals, and it can be shown that their radii are multiples of the radius of this sphere. Threshold devices will be provided for eliminating signals originating at such spheres.

FIG. 4 illustrates an arrangement according to the invention. It is carried by the target and comprises two transmitters 1 and 2, transmitting on frequencies F and F respectively. These two transmitters feed the transmitter aerial 3 and transmit continuous wave signals.

A receiver aerial 4 receives the signal from the missile at frequencies P and F the two signals being affected by the Doppler effect.

A threshold mixer 5 receives directly the signals having frequencies F and F and, through aerial 4, the signals having frequencies F and F A local oscillator 6 of frequency F translates the beat frequencies F F 1D and F -F into immediate frequencies.

The system includes two stages, respectively comprising intermediate frequency amplifiers 11 and 12, detectors 21 and 22, clipping amplifiers 31 and 32, and differentiatots 41 and 42, which are connected in series. An AND- gate 7, which receives the signal derived from the two clippers 41 and 42, feeds a counter 8 which is connected, for example, to transmitter 1.

The system operates as follows: the threshold mixer starts providing output signals only when the missile is at a distance only slightly higher than R. The intermediate frequency output signals of frequencies h-f fg-f include the phase information contained in signals having the frequencies F and F In fact this threshold mixer comprises, as known, a conventional mixer, associated with a threshold clipper, having a predetermined output level. If the output signal is lower than this level, no signal appears at the output of the device.

Beat frequency signal f -f is then demodulated in demodulator 21 which delivers at its output a Doppler signal (p such as that illustrated in FIG. 2a. This signal is clipped by amplifier 31 to provide signal E shown in FIG. 5 and differentiated by differentiator 41 to provide signal F The same procedure takes place in the second stage which provides signal F Thus, at the input of gate 7, pulses F and F occur upon cancellation of the corresponding Doppler signals. The signals first coincide at the inputs of gate 7 when the missile is anywhere on the virtual sphere centered at 0 and having a radius R as shown in FIGURE 1. The coincidences occurring at higher distances from the target are not noticed, due to the action of the threshold mixer 5. Gate 7 starts a counter 8 which counts the pulses derived from ditferentiator 41, i.e. the pulses which occur at each cancellation of the Doppler signal. In other terms, a pulse is produced each time the missile passes a sphere in FIG. 3. The counter is stopped at the second coincidence, i.e. when the missile will have left the sphere of 4- radius R. Such counters are well known in the art. Thus, the number of spheres crossed by the missile will indicate, to within the minimum distance thereof from the target. Once the count of the performed counter has been performed, its information may be transmitted by means of one of transmitters 1 or 2.

Since the minimum error is the accuracy of the measurement will of course depend on the choice of the wave length A Alternatively, two pulse counters may be used instead of one, thus substantially reducing the systematic error, since, in this manner, the position of the missile is encompassed by two spheres in each set of spheres. It may be noted that when the distance from the missile to the target is equal to about the two Doppler signals are in quadrature; )\1 and A being close to each other, the distance between two adjacent spheres corresponding, one to M, the other to A may be as small as F and F being the respective frequencies of the two transmitters.

If r varies by Ar, this will result in a variation of f by i.e. when the missile passes from one sphere to the following, since Liki with T being the period and F the mean value of F and F one has F1 F2 4F and AP-rr F,

The difference of position in time Ar between the pulses corresponding to the passage through zero of both Doppler signals will then be given by the following relation ta and f being respectively the angular frequency and the mean frequency of both Doppler signals, the respective frequencies and angular frequencies of which are very close to one another. Thus In order to determine the coincidences, the AND-gate 7 must be capable of sensing the shift between two pulses spaced by AT; therefore the width of the pulses must be smaller than A'r.

In one system of the above type, the error allowed being 40 cm., one had The radius R of the sphere was selected to be equal to 50 m. and the Doppler signals had to be opposite in phase for this distance.

and

Assuming the relative velocity of the missile to be 1500 m./s., then FB= f=2000 c./s.

and AT=2 ,usec.

Since the aerials must detect the target along any direction, they will be of the omni-directional type. Simple calculation based on the radar equation, assuming that the reflecting surface of the missile is 0.1 m. shows that the transmitted power must be of the order of 5 to 80 mw., according to the radius R.

FIGS. 6 and 7, where the same reference numerals designate the same elements as those in FIG. 4 show two modifications.

In FIG. 6 the AND-gate feeds a separate transmitter 8 which transmits to ground the information, as to the instantaneous position of the missile.

In the embodiment illustrated in FIG. 7, the unit is mounted on the missile. Selector 7 is used for controlling a firing device or a proximity fuze 10, as soon as the missile is at a predetermined distance R from the target.

It is to be understood that the invention is not limited to the embodiments shown, which are given only by way of example.

What is claimed, is:

l. A system for measuring the instantaneous distance, between two objects in relative motion with respect to each other comprising, carried by one of said objects: two transmitters respectively transmitting ultra-high frequency signals on two frequencies close to each other; a receiver for directly receiving said two signals and for receiving the respective echoes of said signals transmitted by said other object; means for detecting the respective Doppler frequency signals of said echo signals; means for deriving respective pulses each time a Doppler signal is equal to zero; and AND-gate having two inputs for receiving respectively said pulses and an output; and a counter coupled to said output and thus controlled by said AND-gate for starting upon first simultaneous reception of said two pulses and stopping upon second simultaneous reception of said pulses.

2. A system for measuring the instantaneous distance, between two objects in relative motion with respect to each other, comprising, carried by one of said objects: two transmitters respectively transmitting ultra-high frequency signals on two frequencies close to each other; a receiver for directly receiving said two signals and for receiving the respective echoes of said signals transmitted by said other object; means for detecting the respective Doppler frequency signals of said echo signals; means for clipping said received signals at a predetermined threshold level; means for deriving respective pulses each time a Doppler signal is equal to zero; an AND-gate having two inputs for receiving respectively said pulses and an output; and a counter coupled to said output and thus controlled by said AND-gate for starting upon first simultaneous reception of said two pulses, and stopping upon second simultaneous reception of said pulses.

3. A system for measuring the instantaneous distance between two objects in relative motion with respect to each other comprising, carried by one of said objects: two transmitters respectively transmitting ultra-high frequency signals on two frequencies close to each other; a receiver for directly receiving said two signals and for receiving the respective echoes of said signals transmitted by said other object; means for detecting the respective Doppler frequency signals of said echo signals; a thresh old mixer, for detecting, over a predetermined threshold level, the respective Doppler frequencies of said echo signals; two detectors, two clipping signal amplifiers, two differentiators, connected in series to said mixer, for respectively deriving from said Doppler signals two respective series of pulses, occurring each time the corresponding Doppler signal is equal to zero; and AND-gate having two inputs for receiving respectively said pulses and an output; and a counter coupled to said output and thus controlled by said AND-gate for starting upon first simultaneous reception of said two pulses and stopping upon second simultaneous reception of said pulses.

4. A system for measuring the instantaneous distance, between two objects in relative motion with respect to each other comprising, carried by one of said objects; two transmitters respectively transmitting ultra-high frequency signals on two frequencies close to each other; a receiver for directly receiving said two signals and for re ceiving the respective echoes of said signals transmitted by said other object; means for detecting the respective Doppler frequency signals of said echo signals; a threshold mixer, for detecting, over a predetermined threshold level, the respective Doppler frequencies of said echo signals; two detectors, two clipping signal amplifiers, two differentiators connected in series to said mixer, for respectively deriving from said Doppler signals two respective series of pulses, occurring each time the corresponding Doppler signal is equal to zero; an AND-gate having two inputs for receiving respectively said pulses and an output; a counter coupled to said output and thus controlled by said AND-gate for starting upon first simultaneous reception of said two pulses and stopping upon second simultaneous reception of said pulses; and means for coupling said counter to one of said transmitters.

5. A system for measuring the instantaneous distance, between two objects in relative motion with respect to each other comprising carried by one of said objects; two transmitters respectively transmitting ultra-high frequency signals on two frequencies close to each other; a receiver for directly receiving said two signals and for receiving the respective echoes of said signals transmitted by said other object; means for detecting the respective Doppler frequency signals of said echo signals; a threshold mixer, for detecting, over a predetermined threshold level, the respective Doppler frequencies of said echo signals; two detectors, two clipping signal amplifiers, two differentiators, connected in series to said mixer, for respectively deriving from said Doppler signals two respective series of pulses, occurring each time the corresponding Doppler signal is equal to zero; an AND-gate having two inputs for receiving respectively said pulses and an output; a counter coupled to said output and thus controlled by said AND-gate for starting upon first simultaneous reception of said two pulses and stopping upon second simultaneous reception of said pulses; a firing device; and means for connecting said counter to said firing device, for controlling it when the distance between said objects reaches a predetermined value.

6. A system for measuring the instantaneous distance, between two objects in relative motion with respect to each other comprising carried by one of said objects; two transmitters respectively transmitting ultra-high frequency signals on two frequencies close to each other; a receiver for directly receiving said two signals and for reing two inputs for receiving respectively said pulses and an output; a counter coupled to said output and thus controlled by said AND-gate for starting upon first simultaneous reception of said two pulses and stopping upon second simultaneous reception of said pulses, a third transmitter; and means for controlling said second transmitter by said counter, for constantly transmitting an information concerning the instantaneous distance between said two objects.

No references cited. 

1. A SYSTEM FOR MEASURING THE INSTANTANEOUS DISTANCE, BETWEEN TWO OBJECTS IN RELATIVE MOTION WITH RESPECT TO EACH OTHER COMPRISING, CARRIED BY ONE OF SAID OBJECTS: TWO TRANSMITTERS RESPECTIVELY TRANSMITTING ULTRA-HIGH FREQUENCY SIGNALS ON TWO FREQUENCIES CLOSE TO EACH OTHER; A RECEIVER FOR DIRECTLY RECEIVING SAID TWO SIGNALS AND FOR RECEIVING THE RESPECTIVE ECHOES OF SAID SIGNALS TRANSMITTED BY SAID OTHER OBJECT; MEANS FOR DETECTING THE RESPECTIVE DOPPLER FREQUENCY SIGNALS OF SAID ECHO SIGNALS; MEANS FOR DERIVING RESPECTIVE PULSES EACH TIME A DOPPLER SIGNAL IS EQUAL TO ZERO; AND AND-GATE HAVING TWO INPUTS FOR RECEIVING RESPECTIVELY SAID PULSES AND AN OUTPUT; AND A COUNTER COUPLED TO SAID OUTPUT AND THUS CONTROLLED BY SAID AND-GATE FOR STARTING UPON FIRST SIMULTANEOUS RECEPTION OF SAID TWO PULSES AND STOPPING UPON SECOND SIMULTANEOUS RECEPTION OF SAID PULSES. 